It has been known for years to use oil as a quench medium in the heat treating of fabricated metal products. Oil exhibits a particularly desirable cooling response; i.e., a response which is characterized by a relatively fast cooling rate in the high temperature range and a relatively slow cooling rate in the low temperature range. The fast cooling rate in the high range is necessary to avoid the knee of the time-temperature transformation curve and the slow rate in the lower temperature range is desirable to minimize internal stresses in the quenched parts.
Although oil exhibits this highly advantageous cooling response, the secondary characteristics of the material are undesirable. The combustibility of oil leads to frequent fires in heat treating plants, thus affecting worker safety and insurance rates. The polluting characteristics of oil are such as to require special and expensive disposal procedures.
Accordingly, a substantial market has developed for quenchants comprising water soluble polymers such as polyvinyl alcohol, polyglycols, polyvinylpyrrolidone and sodium polyacrylate, the most popular of these being the polyglycols and polyvinylpyrrolidone.
U.S. Pat. No. 3,902,929 "Water Based Quenching Composition Comprising Polyvinylpyrrolidone and Method of Quenching" granted Sept. 2, 1975 to Anthony G. Meszaros discloses the use of an aqueous solution of polyvinylpyrrolidone having an average molecular weight of between about 5,000 and about 400,000 and in concentrations of between about 2% and 10% polymer solids by weight as a quenching composition for metal parts. According to the patent, the preferred molecular weight range is about 50,000 to about 360,000 and further, best results are obtained with an average molecular weight between about 100,000 and 200,000.
The patent further states that suitable results are obtained using a concentration of polyvinylpyrrolidone of about 1 to 13% by weight of the quenching composition.
The patent further discloses the use of rust inhibiting agents and bacteriocidal agents in the bath, but only in very small amounts.